1. Field of the Invention
The present invention relates to an apparatus for flexible SONET access and transmission; and, in particular, to an apparatus for flexible SONET access and transmission which flexibly and economically supports services from voice frequency (POTS) to OC 48 (2488 Mbit/s) with the potential for value added services, mixed ATM and STM multiplexing; and capable of modular system growth.
2. Description of the Related Art
Optical fibers provide a high bandwidth medium for data transmission. Consequently, optical fibers have found applications in many computer networks, including those used in digital telephone systems. To allow a uniform interface for voice and computer equipment on an integrated voice and computer network using optical fibers, American National Standards, Inc. adopted a standard, known as SONET (Synchronous Optical Network). The SONET standard is described in xe2x80x9cAmerican National Standard for Telecommunications-Digital Hierarchy-Optical Interface Rates and Formats Specification (SONET)xe2x80x9d (xe2x80x9cSONET documentxe2x80x9d), which is hereby incorporated by reference in its entirety. The SONET document defines a hierarchy of data formats to support a layered communication architecture, which comprises the photonic, section, line and path layers. A schematic model of the layered architecture is provided in FIG. 12. Each of these layers, except the photonic layer, builds on services provided by the next lower layer.
The basic data unit of the SONET standard is represented by a frame, called the STS-1 frame, consisting of 90 xe2x80x9ccolumnsxe2x80x9d and 9 xe2x80x98rowsxe2x80x9d of 8-bit bytes. The STS-1 frame is shown graphically in FIG. 13. Under the fixed transmission rate, the STS-1 frame is transmitted in 125 microseconds. Under the SONET standard, as shown in FIG. 13, data of an STS-1 frame is transmitted row by row, and from left to right. In each byte, the most significant bit is transmitted first.
To support the layered architecture, the first three columns of the STS-1 frame are used for carrying transport overhead information, and the remaining 87 columns of the frame, known as the STS-1 Synchronous Payload Envelope (SPE), carry the data to be transported. Path layer overhead are also carried in the STS-1. FIG. 14 shows the allocation of the transport and path overheads in the STS-1 frame. A description of each of the overhead bytes is provided in the SONET document and is therefore omitted from this discussion.
The SONET standard also defines (i) data formats which are each smaller than an STS-1 frame and transported within the STS-1 SPE, called virtual tributaries (VT); and (ii) data formats, designated as STS-N frames (where N is an integer), which are each larger than a STS-1 frame. An STS-N frame is formed by byte inter-leaving N STS-1 frames. The counterparts of the STS-1 and STS-N data formats in the optical fibers are called OC-1 and OC-N (optical carrier level 1 and optical carrier level N) respectively. OC-1 and OC-N are obtained by optical conversions of the respective STS signals after scrambling.
A rough description for each of the layers in the SONET architecture is provided here to facilitate understanding of the present invention. The photonic layer provides transport of bits at a fixed bit rate (Nxc3x9751.84 megabits/second, where N is an integer) across the physical medium, i.e. the optical fibers. The main function of the photonic layer is the conversion between the STS signals and the OC signals.
The section layer deals with the transport of an STS-N frame across the physical medium. In this layer, framing, scrambling, section error monitoring are provided. Equipment which terminates in the section layer reads, interprets and modifies the section overhead bytes of the STS-1 frame.
The line layer deals with the reliable transport of the path layer payload. A path is a basic unit of logical point-to-point connection between equipment providing a service on the network. More than one path layer payload, each typically having a data rate less than the STS-1 basic data rate, can share an STS-SPE. The line layer synchronizes and multiplexes for the path layer. The overhead bytes for the line layer includes overhead involved in maintenance and protection (i.e. error recovery and redundancy) purposes. Equipment which terminates in the line layer reads, interprets and modifies the line layer overhead bytes of the STS-1 frame.
The path layer deals with the transport of services between path terminating equipment. Examples of such services include synchronous and asynchronous DS-1 services and video signals. The main function of the path layer is to map the services into the format required by the line layer.
Previous generation SONET equipment had one or more of the following limitations. The locations in which different types of tributary interface units (e.g., DS1, DS3, or optical interface units) was typically restricted. This lead to inefficiencies in using all of the unit slots in a shelf for different service mixes. Those few systems that allowed a more universal slot usage did not allow for small incremental growth of the lower-rate tributary interfaces. For example, placing 14 DS1s on a working unit and using an identical unit for 1:1 protection. Previous equipment lacked a modular manner in which to increase the capacity of a single shelf system without duplicating all of the common units in the additional shelf. In most cases, the additional shelf had to be a separate network element within the SONET network. Previous generation equipment also typically required many different types of common units to perform such functions as system control, external maintenance LAN network interface, high-speed optical interface, system timing generation, time slot interchange (TSI), and intermediate SONET signal processing. Previous systems used dedicated buses for synchronous transfer mode (STM) and asynchronous transfer mode (ATM) PCM signals with no sharing of the two formats within the same STS-N high-speed multiplexed signal. Typically, ATM and STM signals have been processed in separate, unique shelves. Previous systems did not allow for units to use the tributary interface slots to provide a common processing function across part or all of the system""s PCM data without using add/drop time slots on the PCM buses. Also, previous generation equipment had no provision for a local area network among the tributary interface units that allows for packet processing (e.g., IP store and forward) of data packets within the tributary PCM data. Lastly, typical systems of the prior art terminated dropped paths from both directions of a ring configuration on a single unit and also on its protection partner unit, thus requiring twice as much termination circuitry as necessary.
What is needed is an architecture which allows a more universal SONET access and transmission system that can economically serve both small and large bandwidth applications with an extremely wide range of services and which has the potential for value-added services wherein all high-speed interface and TSI functions are combined onto the same unit (in conjunction with the system backplane).
The present invention provides hardware architecture for a flexible transmission and access platform. The primary high speed interfaces can be SONET STS-1 (52 Mb/s), OC-3 (155 Mb/s), OC-12 (622 Mb/s), or OC-48 (2488 Mb/s). For OC-3, OC-12, and OC-48, line terminal, linear add/drop multiplex, and unidirectional path switched ring network topologies are supported. On the tributary input side, services between voice frequency plain-old telephone service (POTS) and OC-3 can be supported.
The time slot interchange function is performed on the backplane so that the system can simultaneously and economically support STM time slot interchange (TSI) of channels from 16 kb/s through 51 Mb/s as well as ATM cell multiplexing. Systems using an integrated circuit for all TSI and cell multiplex functions typically require a different, costly device for narrowband services (16-384 kb/s), wideband services (1.5-50 Mb/s), and ATM cells.
A single, small shelf is the building block for the SONET access and transmission system of the present invention. The xe2x80x9cprimaryxe2x80x9d shelf can support up to 84 DS1 interfaces, up to 168 POTS interfaces, or a mix of narrowband and wideband services. When the tributary interface units in the system use the entire bandwidth of the primary high speed interface, a second shelf can be subtended from the primary shelf in one of two ways. First, a single shelf can be subtended as an xe2x80x9cexpansionxe2x80x9d shelf in which there is minimal replication of common units. The expansion interface units in the expansion shelf are the only common units and have the minimum intelligence necessary to provide a buffered interface for PCM signals between the tributary units in the expansion shelf and the high speed interface unit in the primary shelf. The second method for subtending an additional shelf is through a ring/bus connection that allows an economical chaining of multiple shelves together in the same location. In this application, the only units unique to the subtended shelves are the intershelf ring interface units, which provide the PCM data transfer between the shelf backplane and the intershelf bus and limited shelf control.
Within the shelves, PCM buses are preferably partitioned into 4-bit wide parallel buses with a xe2x80x9cbyte statusxe2x80x9d signal per bus. This choice of bus width is highly efficient. Three sets of transmit and receive PCM buses connect to each tributary interface card slot. In normal operation, each bus carries an STS-1 bandwidth. One transmit/receive bus pair is accessible by all tributary slots. The other two transmit/receive bus pairs are partitioned such that two different physical sets of buses service two groups of tributary units.
This segmenting allows either having all tributary units share the same logical bus, or to use the one group of tributary unit slots for other higher-speed applications or common data processing functions.
Multiple data communications buses exist in the backplane to provide common control communications for the shelf, SONET data communication channels, and a LAN for value-added processing of the user data.
The tributary unit connectors and signal assignments allow cost-sensitive narrowband units to use a low-cost connector and bus interface that is a subset of the backplane connector.
Accordingly, A SONET network interface for interconnecting at least one high speed unit (HSU) with at least two low speed interface units (LSUs) to enable transmission of signals therebetween is provided. The interface comprises: a common bus of predetermined bit width for interfacing the at least one HSU unit with each of the at least two LSU units to enable transmission of signals from each of the at least two LSUs to the at least one HSU, and reception of the signals from the at least one HSU to each of the at least two LSUs; a first partition bus of the predetermined bit width for interfacing the at least one HSU to a predetermined number of the at least two LSUs, the first partition bus being partitioned into a first bus for interfacing the at least one HSU to a first subset of the at least two LSUs and a second bus for interfacing the at least one HSU to a second subset of the at least two LSUs; and a second partition bus of the predetermined bit width for interfacing the at least one HSU to a predetermined number of the at least two LSU units, the second partition bus being partitioned into a third bus for interfacing the at least one HSU unit to the first subset of the at least two LSUs and a fourth bus for interfacing the at least one HSU unit with the second subset of the at least two LSUs. Wherein the first, second, third, and fourth busses of respective first and second partition buses have a bit width of at least the same size as the predetermined bit width to enable increased flexibility of signal transmissions to and from a SONET network. Preferably, the first and second partition buses have a bit width that is the same size as the predetermined bit width.
Also provided is a method for interconnecting at least one HSU with at least two LSUs in a SONET network interface to enable transmission of signals between the HSU and LSUs. The method comprises the steps of: interfacing the at least one HSU unit with each of the at least two LSU units by means of a common bus of predetermined bit width to enable transmission of signals from each of the at least two LSUs to the at least one HSU, and reception of the signals from the at least one HSU to each of the at least two LSUs: interfacing the at least one HSU to a predetermined number of the at least two LSUs by means of a first partition bus of the predetermined bit width for , the first partition bus being partitioned into a first bus for interfacing the at least one HSU to a first subset of the at least two LSUs and a second bus for interfacing the at least one HSU to a second subset of the at least two LSUs; and interfacing the at least one HSU to a predetermined number of the at least two LSU units by means of a second partition bus of the predetermined bit width, the second partition bus being partitioned into a third bus for interfacing the at least one HSU unit to the first subset of the at least two LSUs and a fourth bus for interfacing the at least one HSU unit with the second subset of the at least two LSUs, Whereby the first, second, third, and fourth busses of respective first and second partition buses have a bit width of at least the same size as the predetermined bit width to enable increased flexibility of signal transmissions to and from a SONET network. Preferably, the first and second partition buses have a bit width that is the same size as the predetermined bit width.